Tablet sizes vary. There are a couple of 7", not sure about where the iPad mini falls. The regular iPad is not as big as my Acer A500. That tablet is 10 1/2 inches. There are patterns all over the internet, Whistle Pig has a few, and you can check out the average sizes. Go look where they sell various tablets, Office Depot has a few different ones.
The Nexus 7 & 10 inches are popular, so is the ipad, not sure about the mini. Samsung has a 7 and 10" also.
The Windows Surface seems like it's bigger. Have not seen a pattern for a case for that, yet.
While the 10 1/2 might be big, you can fit an iPad in it.
The basic Nook and Kindle are small. The larger ones will fit the 7" bag.